27.2. The Statistics Collector

Postgres Pro's statistics collector is a subsystem that supports collection and reporting of information about server activity. Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. It can also count calls to user-defined functions and the total time spent in each one.

Postgres Pro also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. This facility is independent of the collector process.

27.2.1. Statistics Collection Configuration

Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. This is controlled by configuration parameters that are normally set in postgresql.conf. (See Chapter 18 for details about setting configuration parameters.)

The parameter track_activities enables monitoring of the current command being executed by any server process.

The parameter track_counts controls whether statistics are collected about table and index accesses.

The parameter track_functions enables tracking of usage of user-defined functions.

The parameter track_io_timing enables monitoring of block read and write times.

Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.)

The statistics collector transmits the collected information to other Postgres Pro processes through temporary files. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. When the server shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. When recovery is performed at server start (e.g. after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset.

27.2.2. Viewing Statistics

Several predefined views, listed in Table 27.1, are available to show the current state of the system. There are also several other views, listed in Table 27.2, available to show the results of statistics collection. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section 27.2.3.

When using the statistics to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). So the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date.

Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. So the statistics will show static information as long as you continue the current transaction. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. But if you want to see new results with each query, be sure to do the queries outside any transaction block. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). The next use of statistical information will cause a new snapshot to be fetched.

A transaction can also see its own statistics (as yet untransmitted to the collector) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction.

Table 27.1. Dynamic Statistics Views

View NameDescription
pg_stat_activity One row per server process, showing information related to the current activity of that process, such as state and current query. See pg_stat_activity for details.
pg_stat_replicationOne row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details.
pg_stat_wal_receiverOnly one row, showing statistics about the WAL receiver from that receiver's connected server. See pg_stat_wal_receiver for details.
pg_stat_subscriptionAt least one row per subscription, showing information about the subscription workers. See pg_stat_subscription for details.
pg_stat_sslOne row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details.
pg_stat_progress_vacuumOne row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See Section 27.4.1.

Table 27.2. Collected Statistics Views

View NameDescription
pg_stat_archiverOne row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details.
pg_stat_bgwriterOne row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details.
pg_stat_databaseOne row per database, showing database-wide statistics. See pg_stat_database for details.
pg_stat_database_conflicts One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details.
pg_stat_all_tables One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details.
pg_stat_sys_tablesSame as pg_stat_all_tables, except that only system tables are shown.
pg_stat_user_tablesSame as pg_stat_all_tables, except that only user tables are shown.
pg_stat_xact_all_tablesSimilar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in pg_stat_all_tables and related views). The columns for numbers of live and dead rows and vacuum and analyze actions are not present in this view.
pg_stat_xact_sys_tablesSame as pg_stat_xact_all_tables, except that only system tables are shown.
pg_stat_xact_user_tablesSame as pg_stat_xact_all_tables, except that only user tables are shown.
pg_stat_all_indexes One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details.
pg_stat_sys_indexesSame as pg_stat_all_indexes, except that only indexes on system tables are shown.
pg_stat_user_indexesSame as pg_stat_all_indexes, except that only indexes on user tables are shown.
pg_statio_all_tables One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details.
pg_statio_sys_tablesSame as pg_statio_all_tables, except that only system tables are shown.
pg_statio_user_tablesSame as pg_statio_all_tables, except that only user tables are shown.
pg_statio_all_indexes One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details.
pg_statio_sys_indexesSame as pg_statio_all_indexes, except that only indexes on system tables are shown.
pg_statio_user_indexesSame as pg_statio_all_indexes, except that only indexes on user tables are shown.
pg_statio_all_sequences One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details.
pg_statio_sys_sequencesSame as pg_statio_all_sequences, except that only system sequences are shown. (Presently, no system sequences are defined, so this view is always empty.)
pg_statio_user_sequencesSame as pg_statio_all_sequences, except that only user sequences are shown.
pg_stat_user_functions One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details.
pg_stat_xact_user_functionsSimilar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions).

The per-index statistics are particularly useful to determine which indexes are being used and how effective they are.

The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. When the number of actual disk reads is much smaller than the number of buffer hits, then the cache is satisfying most read requests without invoking a kernel call. However, these statistics do not give the entire story: due to the way in which Postgres Pro handles disk I/O, data that is not in the Postgres Pro buffer cache might still reside in the kernel's I/O cache, and might therefore still be fetched without requiring a physical read. Users interested in obtaining more detailed information on Postgres Pro I/O behavior are advised to use the Postgres Pro statistics collector in combination with operating system utilities that allow insight into the kernel's handling of I/O.

Table 27.3. pg_stat_activity View

ColumnTypeDescription
datidoidOID of the database this backend is connected to
datnamenameName of the database this backend is connected to
pidintegerProcess ID of this backend
usesysidoidOID of the user logged into this backend
usenamenameName of the user logged into this backend
application_nametextName of the application that is connected to this backend
client_addrinetIP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum.
client_hostnametextHost name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.
client_portintegerTCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used
backend_starttimestamp with time zoneTime when this process was started. For client backends, this is the time the client connected to the server.
xact_starttimestamp with time zoneTime when this process' current transaction was started, or null if no transaction is active. If the current query is the first of its transaction, this column is equal to the query_start column.
query_starttimestamp with time zoneTime when the currently active query was started, or if state is not active, when the last query was started
state_changetimestamp with time zoneTime when the state was last changed
wait_event_typetextThe type of event for which the backend is waiting, if any; otherwise NULL. Possible values are:
  • LWLock: The backend is waiting for a lightweight lock. Each such lock protects a particular data structure in shared memory. wait_event will contain a name identifying the purpose of the lightweight lock. (Some locks have specific names; others are part of a group of locks each with a similar purpose.)

  • Lock: The backend is waiting for a heavyweight lock. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. wait_event will identify the type of lock awaited.

  • BufferPin: The server process is waiting to access to a data buffer during a period when no other process can be examining that buffer. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question.

  • Activity: The server process is idle. This is used by system processes waiting for activity in their main processing loop. wait_event will identify the specific wait point.

  • Extension: The server process is waiting for activity in an extension module. This category is useful for modules to track custom waiting points.

  • Client: The server process is waiting for some activity on a socket from user applications, and that the server expects something to happen that is independent from its internal processes. wait_event will identify the specific wait point.

  • IPC: The server process is waiting for some activity from another process in the server. wait_event will identify the specific wait point.

  • Timeout: The server process is waiting for a timeout to expire. wait_event will identify the specific wait point.

  • IO: The server process is waiting for a IO to complete. wait_event will identify the specific wait point.

wait_eventtextWait event name if backend is currently waiting, otherwise NULL. See Table 27.4 for details.
statetextCurrent overall state of this backend. Possible values are:
  • active: The backend is executing a query.

  • idle: The backend is waiting for a new client command.

  • idle in transaction: The backend is in a transaction, but is not currently executing a query.

  • idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error.

  • fastpath function call: The backend is executing a fast-path function.

  • disabled: This state is reported if track_activities is disabled in this backend.

backend_xidxidTop-level transaction identifier of this backend, if any.
backend_xminxidThe current backend's xmin horizon.
querytextText of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed. By default the query text is truncated at 1024 characters; this value can be changed via the parameter track_activity_query_size.
backend_typetextType of current backend. Possible types are autovacuum launcher, autovacuum worker, background worker, background writer, client backend, checkpointer, startup, walreceiver, walsender and walwriter.

The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.

Note

The wait_event and state columns are independent. If a backend is in the active state, it may or may not be waiting on some event. If the state is active and wait_event is non-null, it means that a query is being executed, but is being blocked somewhere in the system.

Table 27.4. wait_event Description

Wait Event TypeWait Event NameDescription
LWLockShmemIndexLockWaiting to find or allocate space in shared memory.
OidGenLockWaiting to allocate or assign an OID.
XidGenLockWaiting to allocate or assign a transaction id.
ProcArrayLockWaiting to get a snapshot or clearing a transaction id at transaction end.
SInvalReadLockWaiting to retrieve or remove messages from shared invalidation queue.
SInvalWriteLockWaiting to add a message in shared invalidation queue.
WALBufMappingLockWaiting to replace a page in WAL buffers.
WALWriteLockWaiting for WAL buffers to be written to disk.
ControlFileLockWaiting to read or update the control file or creation of a new WAL file.
CheckpointLockWaiting to perform checkpoint.
CLogControlLockWaiting to read or update transaction status.
SubtransControlLockWaiting to read or update sub-transaction information.
MultiXactGenLockWaiting to read or update shared multixact state.
MultiXactOffsetControlLockWaiting to read or update multixact offset mappings.
MultiXactMemberControlLockWaiting to read or update multixact member mappings.
RelCacheInitLockWaiting to read or write relation cache initialization file.
CheckpointerCommLockWaiting to manage fsync requests.
TwoPhaseStateLockWaiting to read or update the state of prepared transactions.
TablespaceCreateLockWaiting to create or drop the tablespace.
BtreeVacuumLockWaiting to read or update vacuum-related information for a B-tree index.
AddinShmemInitLockWaiting to manage space allocation in shared memory.
AutovacuumLockAutovacuum worker or launcher waiting to update or read the current state of autovacuum workers.
AutovacuumScheduleLockWaiting to ensure that the table it has selected for a vacuum still needs vacuuming.
SyncScanLockWaiting to get the start location of a scan on a table for synchronized scans.
RelationMappingLockWaiting to update the relation map file used to store catalog to filenode mapping.
AsyncCtlLockWaiting to read or update shared notification state.
AsyncQueueLockWaiting to read or update notification messages.
SerializableXactHashLockWaiting to retrieve or store information about serializable transactions.
SerializableFinishedListLockWaiting to access the list of finished serializable transactions.
SerializablePredicateLockListLockWaiting to perform an operation on a list of locks held by serializable transactions.
OldSerXidLockWaiting to read or record conflicting serializable transactions.
SyncRepLockWaiting to read or update information about synchronous replicas.
BackgroundWorkerLockWaiting to read or update background worker state.
DynamicSharedMemoryControlLockWaiting to read or update dynamic shared memory state.
AutoFileLockWaiting to update the postgresql.auto.conf file.
ReplicationSlotAllocationLockWaiting to allocate or free a replication slot.
ReplicationSlotControlLockWaiting to read or update replication slot state.
CommitTsControlLockWaiting to read or update transaction commit timestamps.
CommitTsLockWaiting to read or update the last value set for the transaction timestamp.
ReplicationOriginLockWaiting to setup, drop or use replication origin.
MultiXactTruncationLockWaiting to read or truncate multixact information.
OldSnapshotTimeMapLockWaiting to read or update old snapshot control information.
BackendRandomLockWaiting to generate a random number.
LogicalRepWorkerLockWaiting for action on logical replication worker to finish.
CLogTruncationLockWaiting to execute txid_status or update the oldest transaction id available to it.
clogWaiting for I/O on a clog (transaction status) buffer.
commit_timestampWaiting for I/O on commit timestamp buffer.
subtransWaiting for I/O a subtransaction buffer.
multixact_offsetWaiting for I/O on a multixact offset buffer.
multixact_memberWaiting for I/O on a multixact_member buffer.
asyncWaiting for I/O on an async (notify) buffer.
oldserxidWaiting to I/O on an oldserxid buffer.
wal_insertWaiting to insert WAL into a memory buffer.
buffer_contentWaiting to read or write a data page in memory.
buffer_ioWaiting for I/O on a data page.
replication_originWaiting to read or update the replication progress.
replication_slot_ioWaiting for I/O on a replication slot.
procWaiting to read or update the fast-path lock information.
buffer_mappingWaiting to associate a data block with a buffer in the buffer pool.
lock_managerWaiting to add or examine locks for backends, or waiting to join or exit a locking group (used by parallel query).
predicate_lock_managerWaiting to add or examine predicate lock information.
parallel_query_dsaWaiting for parallel query dynamic shared memory allocation lock.
tbmWaiting for TBM shared iterator lock.
LockrelationWaiting to acquire a lock on a relation.
extendWaiting to extend a relation.
pageWaiting to acquire a lock on page of a relation.
tupleWaiting to acquire a lock on a tuple.
transactionidWaiting for a transaction to finish.
virtualxidWaiting to acquire a virtual xid lock.
speculative tokenWaiting to acquire a speculative insertion lock.
objectWaiting to acquire a lock on a non-relation database object.
userlockWaiting to acquire a user lock.
advisoryWaiting to acquire an advisory user lock.
BufferPinBufferPinWaiting to acquire a pin on a buffer.
ActivityArchiverMainWaiting in main loop of the archiver process.
AutoVacuumMainWaiting in main loop of autovacuum launcher process.
BgWriterHibernateWaiting in background writer process, hibernating.
BgWriterMainWaiting in main loop of background writer process background worker.
CheckpointerMainWaiting in main loop of checkpointer process.
LogicalApplyMainWaiting in main loop of logical apply process.
LogicalLauncherMainWaiting in main loop of logical launcher process.
PgStatMainWaiting in main loop of the statistics collector process.
RecoveryWalAllWaiting for WAL from any kind of source (local, archive or stream) at recovery.
RecoveryWalStreamWaiting for WAL from a stream at recovery.
SysLoggerMainWaiting in main loop of syslogger process.
WalReceiverMainWaiting in main loop of WAL receiver process.
WalSenderMainWaiting in main loop of WAL sender process.
WalWriterMainWaiting in main loop of WAL writer process.
ClientClientReadWaiting to read data from the client.
ClientWriteWaiting to write data to the client.
LibPQWalReceiverConnectWaiting in WAL receiver to establish connection to remote server.
LibPQWalReceiverReceiveWaiting in WAL receiver to receive data from remote server.
SSLOpenServerWaiting for SSL while attempting connection.
WalReceiverWaitStartWaiting for startup process to send initial data for streaming replication.
WalSenderWaitForWALWaiting for WAL to be flushed in WAL sender process.
WalSenderWriteDataWaiting for any activity when processing replies from WAL receiver in WAL sender process.
ExtensionExtensionWaiting in an extension.
IPCBgWorkerShutdownWaiting for background worker to shut down.
BgWorkerStartupWaiting for background worker to start up.
BtreePageWaiting for the page number needed to continue a parallel B-tree scan to become available.
ExecuteGatherWaiting for activity from child process when executing Gather node.
LogicalSyncDataWaiting for logical replication remote server to send data for initial table synchronization.
LogicalSyncStateChangeWaiting for logical replication remote server to change state.
MessageQueueInternalWaiting for other process to be attached in shared message queue.
MessageQueuePutMessageWaiting to write a protocol message to a shared message queue.
MessageQueueReceiveWaiting to receive bytes from a shared message queue.
MessageQueueSendWaiting to send bytes to a shared message queue.
ParallelBitmapScanWaiting for parallel bitmap scan to become initialized.
ParallelFinishWaiting for parallel workers to finish computing.
ProcArrayGroupUpdateWaiting for group leader to clear transaction id at transaction end.
ReplicationOriginDropWaiting for a replication origin to become inactive to be dropped.
ReplicationSlotDropWaiting for a replication slot to become inactive to be dropped.
SafeSnapshotWaiting for a snapshot for a READ ONLY DEFERRABLE transaction.
SyncRepWaiting for confirmation from remote server during synchronous replication.
TimeoutBaseBackupThrottleWaiting during base backup when throttling activity.
PgSleepWaiting in process that called pg_sleep.
RecoveryApplyDelayWaiting to apply WAL at recovery because it is delayed.
IOBufFileReadWaiting for a read from a buffered file.
BufFileWriteWaiting for a write to a buffered file.
ControlFileReadWaiting for a read from the control file.
ControlFileSyncWaiting for the control file to reach stable storage.
ControlFileSyncUpdateWaiting for an update to the control file to reach stable storage.
ControlFileWriteWaiting for a write to the control file.
ControlFileWriteUpdateWaiting for a write to update the control file.
CopyFileReadWaiting for a read during a file copy operation.
CopyFileWriteWaiting for a write during a file copy operation.
DataFileExtendWaiting for a relation data file to be extended.
DataFileFlushWaiting for a relation data file to reach stable storage.
DataFileImmediateSyncWaiting for an immediate synchronization of a relation data file to stable storage.
DataFilePrefetchWaiting for an asynchronous prefetch from a relation data file.
DataFileReadWaiting for a read from a relation data file.
DataFileSyncWaiting for changes to a relation data file to reach stable storage.
DataFileTruncateWaiting for a relation data file to be truncated.
DataFileWriteWaiting for a write to a relation data file.
DSMFillZeroWriteWaiting to write zero bytes to a dynamic shared memory backing file.
LockFileAddToDataDirReadWaiting for a read while adding a line to the data directory lock file.
LockFileAddToDataDirSyncWaiting for data to reach stable storage while adding a line to the data directory lock file.
LockFileAddToDataDirWriteWaiting for a write while adding a line to the data directory lock file.
LockFileCreateReadWaiting to read while creating the data directory lock file.
LockFileCreateSyncWaiting for data to reach stable storage while creating the data directory lock file.
LockFileCreateWriteWaiting for a write while creating the data directory lock file.
LockFileReCheckDataDirReadWaiting for a read during recheck of the data directory lock file.
LogicalRewriteCheckpointSyncWaiting for logical rewrite mappings to reach stable storage during a checkpoint.
LogicalRewriteMappingSyncWaiting for mapping data to reach stable storage during a logical rewrite.
LogicalRewriteMappingWriteWaiting for a write of mapping data during a logical rewrite.
LogicalRewriteSyncWaiting for logical rewrite mappings to reach stable storage.
LogicalRewriteWriteWaiting for a write of logical rewrite mappings.
RelationMapReadWaiting for a read of the relation map file.
RelationMapSyncWaiting for the relation map file to reach stable storage.
RelationMapWriteWaiting for a write to the relation map file.
ReorderBufferReadWaiting for a read during reorder buffer management.
ReorderBufferWriteWaiting for a write during reorder buffer management.
ReorderLogicalMappingReadWaiting for a read of a logical mapping during reorder buffer management.
ReplicationSlotReadWaiting for a read from a replication slot control file.
ReplicationSlotRestoreSyncWaiting for a replication slot control file to reach stable storage while restoring it to memory.
ReplicationSlotSyncWaiting for a replication slot control file to reach stable storage.
ReplicationSlotWriteWaiting for a write to a replication slot control file.
SLRUFlushSyncWaiting for SLRU data to reach stable storage during a checkpoint or database shutdown.
SLRUReadWaiting for a read of an SLRU page.
SLRUSyncWaiting for SLRU data to reach stable storage following a page write.
SLRUWriteWaiting for a write of an SLRU page.
SnapbuildReadWaiting for a read of a serialized historical catalog snapshot.
SnapbuildSyncWaiting for a serialized historical catalog snapshot to reach stable storage.
SnapbuildWriteWaiting for a write of a serialized historical catalog snapshot.
TimelineHistoryFileSyncWaiting for a timeline history file received via streaming replication to reach stable storage.
TimelineHistoryFileWriteWaiting for a write of a timeline history file received via streaming replication.
TimelineHistoryReadWaiting for a read of a timeline history file.
TimelineHistorySyncWaiting for a newly created timeline history file to reach stable storage.
TimelineHistoryWriteWaiting for a write of a newly created timeline history file.
TwophaseFileReadWaiting for a read of a two phase state file.
TwophaseFileSyncWaiting for a two phase state file to reach stable storage.
TwophaseFileWriteWaiting for a write of a two phase state file.
WALBootstrapSyncWaiting for WAL to reach stable storage during bootstrapping.
WALBootstrapWriteWaiting for a write of a WAL page during bootstrapping.
WALCopyReadWaiting for a read when creating a new WAL segment by copying an existing one.
WALCopySyncWaiting a new WAL segment created by copying an existing one to reach stable storage.
WALCopyWriteWaiting for a write when creating a new WAL segment by copying an existing one.
WALInitSyncWaiting for a newly initialized WAL file to reach stable storage.
WALInitWriteWaiting for a write while initializing a new WAL file.
WALReadWaiting for a read from a WAL file.
WALSenderTimelineHistoryReadWaiting for a read from a timeline history file during walsender timeline command.
WALSyncMethodAssignWaiting for data to reach stable storage while assigning WAL sync method.
WALWriteWaiting for a write to a WAL file.

Note

For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event. It is quite possible that user has registered the tranche in one of the backends (by having allocation in dynamic shared memory) in which case other backends won't have that information, so we display extension for such cases.

Here is an example of how wait events can be viewed

SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
 pid  | wait_event_type |  wait_event
------+-----------------+---------------
 2540 | Lock            | relation
 6644 | LWLock          | ProcArrayLock
(2 rows)

Table 27.5. pg_stat_replication View

ColumnTypeDescription
pidintegerProcess ID of a WAL sender process
usesysidoidOID of the user logged into this WAL sender process
usenamenameName of the user logged into this WAL sender process
application_nametextName of the application that is connected to this WAL sender
client_addrinetIP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine.
client_hostnametextHost name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.
client_portintegerTCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used
backend_starttimestamp with time zoneTime when this process was started, i.e., when the client connected to this WAL sender
backend_xminxidThis standby's xmin horizon reported by hot_standby_feedback.
statetextCurrent WAL sender state. Possible values are:
  • startup: This WAL sender is starting up.

  • catchup: This WAL sender's connected standby is catching up with the primary.

  • streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary.

  • backup: This WAL sender is sending a backup.

  • stopping: This WAL sender is stopping.

sent_lsnpg_lsnLast write-ahead log location sent on this connection
write_lsnpg_lsnLast write-ahead log location written to disk by this standby server
flush_lsnpg_lsnLast write-ahead log location flushed to disk by this standby server
replay_lsnpg_lsnLast write-ahead log location replayed into the database on this standby server
write_lagintervalTime elapsed between flushing recent WAL locally and receiving notification that this standby server has written it (but not yet flushed it or applied it). This can be used to gauge the delay that synchronous_commit level remote_write incurred while committing if this server was configured as a synchronous standby.
flush_lagintervalTime elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). This can be used to gauge the delay that synchronous_commit level on incurred while committing if this server was configured as a synchronous standby.
replay_lagintervalTime elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby.
sync_priorityintegerPriority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. This has no effect in a quorum-based synchronous replication.
sync_statetextSynchronous state of this standby server. Possible values are:
  • async: This standby server is asynchronous.

  • potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails.

  • sync: This standby server is synchronous.

  • quorum: This standby server is considered as a candidate for quorum standbys.


The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers.

The lag times reported in the pg_stat_replication view are measurements of the time taken for recent WAL to be written, flushed and replayed and for the sender to know about it. These times represent the commit delay that was (or would have been) introduced by each synchronous commit level, if the remote server was configured as a synchronous standby. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. If the standby server has entirely caught up with the sending server and there is no more WAL activity, the most recently measured lag times will continue to be displayed for a short time and then show NULL.

Lag times work automatically for physical replication. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag.

Note

The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. In particular, when the standby has caught up completely, pg_stat_replication shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value.

Table 27.6. pg_stat_wal_receiver View

ColumnTypeDescription
pidintegerProcess ID of the WAL receiver process
statustextActivity status of the WAL receiver process
receive_start_lsnpg_lsnFirst write-ahead log location used when WAL receiver is started
receive_start_tliintegerFirst timeline number used when WAL receiver is started
received_lsnpg_lsnLast write-ahead log location already received and flushed to disk, the initial value of this field being the first log location used when WAL receiver is started
received_tliintegerTimeline number of last write-ahead log location received and flushed to disk, the initial value of this field being the timeline number of the first log location used when WAL receiver is started
last_msg_send_timetimestamp with time zoneSend time of last message received from origin WAL sender
last_msg_receipt_timetimestamp with time zoneReceipt time of last message received from origin WAL sender
latest_end_lsnpg_lsnLast write-ahead log location reported to origin WAL sender
latest_end_timetimestamp with time zoneTime of last write-ahead log location reported to origin WAL sender
slot_nametextReplication slot name used by this WAL receiver
conninfotext Connection string used by this WAL receiver, with security-sensitive fields obfuscated.

The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server.

Table 27.7. pg_stat_subscription View

ColumnTypeDescription
subidoidOID of the subscription
subnametextName of the subscription
pidintegerProcess ID of the subscription worker process
relidOidOID of the relation that the worker is synchronizing; null for the main apply worker
received_lsnpg_lsnLast write-ahead log location received, the initial value of this field being 0
last_msg_send_timetimestamp with time zoneSend time of last message received from origin WAL sender
last_msg_receipt_timetimestamp with time zoneReceipt time of last message received from origin WAL sender
latest_end_lsnpg_lsnLast write-ahead log location reported to origin WAL sender
latest_end_timetimestamp with time zoneTime of last write-ahead log location reported to origin WAL sender

The pg_stat_subscription view will contain one row per subscription for main worker (with null PID if the worker is not running), and additional rows for workers handling the initial data copy of the subscribed tables.

Table 27.8. pg_stat_ssl View

ColumnTypeDescription
pidintegerProcess ID of a backend or WAL sender process
sslbooleanTrue if SSL is used on this connection
versiontextVersion of SSL in use, or NULL if SSL is not in use on this connection
ciphertextName of SSL cipher in use, or NULL if SSL is not in use on this connection
bitsintegerNumber of bits in the encryption algorithm used, or NULL if SSL is not used on this connection
compressionbooleanTrue if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection
clientdntextDistinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build)

The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection.

Table 27.9. pg_stat_archiver View

ColumnTypeDescription
archived_countbigintNumber of WAL files that have been successfully archived
last_archived_waltextName of the last WAL file successfully archived
last_archived_timetimestamp with time zoneTime of the last successful archive operation
failed_countbigintNumber of failed attempts for archiving WAL files
last_failed_waltextName of the WAL file of the last failed archival operation
last_failed_timetimestamp with time zoneTime of the last failed archival operation
stats_resettimestamp with time zoneTime at which these statistics were last reset

The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster.

Table 27.10. pg_stat_bgwriter View

ColumnTypeDescription
checkpoints_timedbigintNumber of scheduled checkpoints that have been performed
checkpoints_reqbigintNumber of requested checkpoints that have been performed
checkpoint_write_timedouble precision Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds
checkpoint_sync_timedouble precision Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds
buffers_checkpointbigintNumber of buffers written during checkpoints
buffers_cleanbigintNumber of buffers written by the background writer
maxwritten_cleanbigintNumber of times the background writer stopped a cleaning scan because it had written too many buffers
buffers_backendbigintNumber of buffers written directly by a backend
buffers_backend_fsyncbigintNumber of times a backend had to execute its own fsync call (normally the background writer handles those even when the backend does its own write)
buffers_allocbigintNumber of buffers allocated
stats_resettimestamp with time zoneTime at which these statistics were last reset

The pg_stat_bgwriter view will always have a single row, containing global data for the cluster.

Table 27.11. pg_stat_database View

ColumnTypeDescription
datidoidOID of a database
datnamenameName of this database
numbackendsintegerNumber of backends currently connected to this database. This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset.
xact_commitbigintNumber of transactions in this database that have been committed
xact_rollbackbigintNumber of transactions in this database that have been rolled back
blks_readbigintNumber of disk blocks read in this database
blks_hitbigintNumber of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the Postgres Pro buffer cache, not the operating system's file system cache)
tup_returnedbigintNumber of rows returned by queries in this database
tup_fetchedbigintNumber of rows fetched by queries in this database
tup_insertedbigintNumber of rows inserted by queries in this database
tup_updatedbigintNumber of rows updated by queries in this database
tup_deletedbigintNumber of rows deleted by queries in this database
conflictsbigintNumber of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.)
temp_filesbigintNumber of temporary files created by queries in this database. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the log_temp_files setting.
temp_bytesbigintTotal amount of data written to temporary files by queries in this database. All temporary files are counted, regardless of why the temporary file was created, and regardless of the log_temp_files setting.
deadlocksbigintNumber of deadlocks detected in this database
blk_read_timedouble precisionTime spent reading data file blocks by backends in this database, in milliseconds
blk_write_timedouble precisionTime spent writing data file blocks by backends in this database, in milliseconds
stats_resettimestamp with time zoneTime at which these statistics were last reset

The pg_stat_database view will contain one row for each database in the cluster, showing database-wide statistics.

Table 27.12. pg_stat_database_conflicts View

ColumnTypeDescription
datidoidOID of a database
datnamenameName of this database
confl_tablespacebigintNumber of queries in this database that have been canceled due to dropped tablespaces
confl_lockbigintNumber of queries in this database that have been canceled due to lock timeouts
confl_snapshotbigintNumber of queries in this database that have been canceled due to old snapshots
confl_bufferpinbigintNumber of queries in this database that have been canceled due to pinned buffers
confl_deadlockbigintNumber of queries in this database that have been canceled due to deadlocks

The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. This view will only contain information on standby servers, since conflicts do not occur on master servers.

Table 27.13. pg_stat_all_tables View

ColumnTypeDescription
relidoidOID of a table
schemanamenameName of the schema that this table is in
relnamenameName of this table
seq_scanbigintNumber of sequential scans initiated on this table
seq_tup_readbigintNumber of live rows fetched by sequential scans
idx_scanbigintNumber of index scans initiated on this table
idx_tup_fetchbigintNumber of live rows fetched by index scans
n_tup_insbigintNumber of rows inserted
n_tup_updbigintNumber of rows updated (includes HOT updated rows)
n_tup_delbigintNumber of rows deleted
n_tup_hot_updbigintNumber of rows HOT updated (i.e., with no separate index update required)
n_live_tupbigintEstimated number of live rows
n_dead_tupbigintEstimated number of dead rows
n_mod_since_analyzebigintEstimated number of rows modified since this table was last analyzed
last_vacuumtimestamp with time zoneLast time at which this table was manually vacuumed (not counting VACUUM FULL)
last_autovacuumtimestamp with time zoneLast time at which this table was vacuumed by the autovacuum daemon
last_analyzetimestamp with time zoneLast time at which this table was manually analyzed
last_autoanalyzetimestamp with time zoneLast time at which this table was analyzed by the autovacuum daemon
vacuum_countbigintNumber of times this table has been manually vacuumed (not counting VACUUM FULL)
autovacuum_countbigintNumber of times this table has been vacuumed by the autovacuum daemon
analyze_countbigintNumber of times this table has been manually analyzed
autoanalyze_countbigintNumber of times this table has been analyzed by the autovacuum daemon

The pg_stat_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about accesses to that specific table. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively.

Table 27.14. pg_stat_all_indexes View

ColumnTypeDescription
relidoidOID of the table for this index
indexrelidoidOID of this index
schemanamenameName of the schema this index is in
relnamenameName of the table for this index
indexrelnamenameName of this index
idx_scanbigintNumber of index scans initiated on this index
idx_tup_readbigintNumber of index entries returned by scans on this index
idx_tup_fetchbigintNumber of live table rows fetched by simple index scans using this index

The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.

Indexes can be used by simple index scans, bitmap index scans, and the optimizer. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale.

Note

The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan.

Table 27.15. pg_statio_all_tables View

ColumnTypeDescription
relidoidOID of a table
schemanamenameName of the schema that this table is in
relnamenameName of this table
heap_blks_readbigintNumber of disk blocks read from this table
heap_blks_hitbigintNumber of buffer hits in this table
idx_blks_readbigintNumber of disk blocks read from all indexes on this table
idx_blks_hitbigintNumber of buffer hits in all indexes on this table
toast_blks_readbigintNumber of disk blocks read from this table's TOAST table (if any)
toast_blks_hitbigintNumber of buffer hits in this table's TOAST table (if any)
tidx_blks_readbigintNumber of disk blocks read from this table's TOAST table indexes (if any)
tidx_blks_hitbigintNumber of buffer hits in this table's TOAST table indexes (if any)

The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively.

Table 27.16. pg_statio_all_indexes View

ColumnTypeDescription
relidoidOID of the table for this index
indexrelidoidOID of this index
schemanamenameName of the schema this index is in
relnamenameName of the table for this index
indexrelnamenameName of this index
idx_blks_readbigintNumber of disk blocks read from this index
idx_blks_hitbigintNumber of buffer hits in this index

The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively.

Table 27.17. pg_statio_all_sequences View

ColumnTypeDescription
relidoidOID of a sequence
schemanamenameName of the schema this sequence is in
relnamenameName of this sequence
blks_readbigintNumber of disk blocks read from this sequence
blks_hitbigintNumber of buffer hits in this sequence

The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence.

Table 27.18. pg_stat_user_functions View

ColumnTypeDescription
funcidoidOID of a function
schemanamenameName of the schema this function is in
funcnamenameName of this function
callsbigintNumber of times this function has been called
total_timedouble precisionTotal time spent in this function and all other functions called by it, in milliseconds
self_timedouble precisionTotal time spent in this function itself, not including other functions called by it, in milliseconds

The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. The track_functions parameter controls exactly which functions are tracked.

27.2.3. Statistics Functions

Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. For details such as the functions' names, consult the definitions of the standard views. (For example, in psql you could issue \d+ pg_stat_activity.) The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The per-table and per-index functions take a table or index OID. The functions for per-function statistics take a function OID. Note that only tables, indexes, and functions in the current database can be seen with these functions.

Additional functions related to statistics collection are listed in Table 27.19.

Table 27.19. Additional Statistics Functions

FunctionReturn TypeDescription
pg_backend_pid()integer Process ID of the server process handling the current session
pg_stat_get_activity(integer)setof record Returns a record of information about the backend with the specified PID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view.
pg_stat_get_snapshot_timestamp()timestamp with time zone Returns the timestamp of the current statistics snapshot
pg_stat_clear_snapshot()void Discard the current statistics snapshot
pg_stat_reset()void Reset all statistics counters for the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others.)
pg_stat_reset_shared(text)void Reset some cluster-wide statistics counters to zero, depending on the argument (requires superuser privileges by default, but EXECUTE for this function can be granted to others). Calling pg_stat_reset_shared('bgwriter') will zero all the counters shown in the pg_stat_bgwriter view. Calling pg_stat_reset_shared('archiver') will zero all the counters shown in the pg_stat_archiver view.
pg_stat_reset_single_table_counters(oid)void Reset statistics for a single table or index in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others)
pg_stat_reset_single_function_counters(oid)void Reset statistics for a single function in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others)

pg_stat_get_activity, the underlying function of the pg_stat_activity view, returns a set of records containing all the available information about each backend process. Sometimes it may be more convenient to obtain just a subset of this information. In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table 27.20. These access functions use a backend ID number, which ranges from one to the number of currently active backends. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. For example, to show the PIDs and current queries of all backends:

SELECT pg_stat_get_backend_pid(s.backendid) AS pid,
       pg_stat_get_backend_activity(s.backendid) AS query
    FROM (SELECT pg_stat_get_backend_idset() AS backendid) AS s;

Table 27.20. Per-Backend Statistics Functions

FunctionReturn TypeDescription
pg_stat_get_backend_idset()setof integerSet of currently active backend ID numbers (from 1 to the number of active backends)
pg_stat_get_backend_activity(integer)textText of this backend's most recent query
pg_stat_get_backend_activity_start(integer)timestamp with time zoneTime when the most recent query was started
pg_stat_get_backend_client_addr(integer)inetIP address of the client connected to this backend
pg_stat_get_backend_client_port(integer)integerTCP port number that the client is using for communication
pg_stat_get_backend_dbid(integer)oidOID of the database this backend is connected to
pg_stat_get_backend_pid(integer)integerProcess ID of this backend
pg_stat_get_backend_start(integer)timestamp with time zoneTime when this process was started
pg_stat_get_backend_userid(integer)oidOID of the user logged into this backend
pg_stat_get_backend_wait_event_type(integer)textWait event type name if backend is currently waiting, otherwise NULL. See Table 27.4 for details.
pg_stat_get_backend_wait_event(integer)textWait event name if backend is currently waiting, otherwise NULL. See Table 27.4 for details.
pg_stat_get_backend_xact_start(integer)timestamp with time zoneTime when the current transaction was started